1. Field of the Invention
The invention relates to alignment of Loran-C navigational systems. Specifically, the method of the invention insures the faithful reproduction of low level digital pulse shape and timing instructions issued by automatic, microprocessor controlled, closed loops when converted to high level radio frequency multipulsed Loran-C navigational signal transmissions.
2. Related Art
The 1947 International Telecommunications Union (ITU) conference at Atlantic City, N.J. set the frequency bandwidth for Loran-C Radio Navigation signals to a 20 kilohertz band, centered about 100 kilohertz. The 1959 Geneva Radio Conference further refined this bandwidth definition for Loran-C radio navigation signals.
The accepted interpretation for all aspects of the Loran-C radio navigation bandwidth allocation is that 99% of the total energy radiated must be within the band of frequencies between 90 kilohertz to 110 kilohertz with no more than 1/2 of one percent above or below the allocated band. This international commitment constrains both the carrier frequency and the pulse shape of all Loran-C radio navigation transmissions throughout the world.
The rise time of a Loran-C pulse should be as fast as possible, within bandwidth constraints, to present the maximum pulse amplitude to a user's receiver prior to arrival of early skywave, which are transmitted signals reflected from the ionosphere. Because skywaves travel further from the transmitter to the receiver than groundwaves, skywaves arrive at the receiver later. The fast rise time minimizes pulse leading edge skywave contamination of the groundwaves used for pulse tracking. Loran-C radio navigation systems are also "multipulsed," radiating more than one pulse per Loran Transmission Interval. Thus, the tail or trailing edge of each radiated pulse must be sufficiently attenuated before beginning the subsequent pulse. Precise control of the radiated pulses' leading edges and controlled attenuation of the pulses' trailing edges are required for a user to unambiguously extract accurate navigational information.
In currently operating Megapulse, Inc. Loran-C equipments, precisely controlled current pulses, which drive a filter-antenna system, resulting in Loran-C pulses which peak about 65 microseconds after time=0. At this point in time a variable damping impedance is switched into the filter to accomplish required trailing edge attenuation. For example, the present generation of Loran-C transmitting equipment, such as that manufactured by Megapulse, Inc., 8 Preston Court, Bedford, Mass. 01730, constructs or synthesizes the Loran-C pulses by supplying drive to only the first four (two positive and two negative) half-cycles of the carrier for each Loran-C pulse. This is accomplished using Drive Half Cycle (DHC) signals. To construct such signals, a plurality of Half Cycle Generator (HCG) units within the Loran-C transmitter each generate a 5 microsecond half cycle current pulse for each Loran-C pulse. A Pulse Amplitude and Timing Control (PATCO) unit defines parallel combinations of Half Cycle Generator (HCG) units, and sets the firing time and amplitude of the individual HCG outputs in response to on-line sampling of a combined HCG output and the Loran-C pulse. Since this closed loop operation requires compliance with desired pulse parameters, alignment of the Loran-C transmitter is critical.
In order for a transmitter set to achieve the required precise leading edge control, manufacturers assume that the transmitter and its associated control circuitry function within design limitations regardless of the number of repairs and associated parts replacements, as long as the repairs are made using parts pretested to the manufacturers' specifications and the repairs are made by qualified personnel. Historically, this has not been the case. It has also been found that under actual operating conditions portions of the low level logic control located in the HCG, are effected by changing the numbers of HCGs being operated in parallel. For example, if the low level logic circuits function properly with 8 HCGs operating on a particular half cycle, the circuitry will not necessarily operate to produce the proper level, when, for example, 24 Half Cycle Generators are operated in parallel. Because much field repair is accomplished by exchanging modules between a suspect HGC and an assumed trouble free unit, this anomaly leads to many false conclusions.
Currently, adjustment of the low level logic circuits within each HCG is normally performed external to the transmitter while a Half Cycle Generator is at a Module Repair Facility. Since each Loran-C transmitter is basically a "one-of-a-kind" custom built unit, there are subtle differences in component tolerances. To insure that precise pulse shape and frequency bandwidth constraints are met, it would be optimum to "customize" the adjustment of the HCG low level logic circuitry by accomplishing the appropriate adjustments with the Half Cycle Generator installed and operating in each individual transmitter set.
To date, no efficient technique has been developed to accomplish this. Manufacturers' alignment procedures require technicians to first determine a "known good circuit board", make closed loop adjustments effecting up to 25 individual HCGs and make comparisons at "standard conditions." Since each adjustment effects all components of a particular closed loop, the manufacturers' approach presents a very difficult set of conditions to meet. This is the area which this invention addresses.